Metered combustion control systems are commonly employed in connection with industrial and commercial boilers for modulating air flow and fuel flow to the burner or burners of the boiler. One type of combustion controller uses air flow and fuel flow actuators to modulate air flow and fuel flow through parallel metering of flows over the entire operating range of the boiler to ensure the safety, efficiency, and environmental requirements of combustion can be satisfied across the entire operating range. In parallel metering control systems, the firing rate demand signal is applied in parallel as the setpoint to two slave flow control loops. One flow control loop monitors fuel flow and the other monitors air flow. The air flow controller controls air flow by manipulating actuators associated with an air damper and/or a variable frequency driver operatively associated with a variable speed air flow fan. The fuel flow controller controls fuel flow by manipulating a fuel actuator, such as a solenoid valve or other type of flow servo valves. The two flow controllers' setpoints are coordinated by an air/fuel flow ratio at the desired firing rate.
The operating range of a boiler is generally defined by its firing range between a low fire point commensurate with the minimum firing rate at which combustion is sustainable and a high fire point commensurate with the maximum energy output of the burner. The firing range depends on the boiler's burner's turndown ratio, that is, the ratio between the highest energy output and the lowest energy output. At each given firing rate within the boiler firing range, a suitable air/fuel flow ratio must be defined, which in turns determines efficiency, emissions and stability of combustion. The determined set of air/fuel flow ratios provides the setpoints for the two flow control loops that is used by the boiler controller during operation of the boiler to modulate the burner fuel valve and the air damper in response to firing rate.
When a combustion control system is first installed on a boiler, the desired air/fuel flow ratios need to be defined at a number of points, i.e. firing rates, within the firing range, because the relationship between the sets of air/fuel flow ratios and firing rates is non-linear. The process of defining the proper air/fuel flow ratios throughout the firing range is commonly referred to as commissioning of the boiler combustion control system. The purpose of the commissioning process is to find a set of coordinated air and fuel flow setpoints (i.e. air/flow ratios) at various points, i.e. firing rates, across the operating range such that safety, efficiency, and environmental requirements can be achieved. During the commissioning process, at each of the respective firing rates at which an optimal set coordinated air and fuel flows is determined, the excess oxygen level and carbon monoxide level associated with combustion at those positions are measured and recorded. Typically, the commissioning process is manually done by a technician and it can be very time consuming.
Generally, the combustion controller includes a first feedback circuit including a pressure controller for adjusting the firing rate in response to a sensed boiler pressure and a second feedback circuit including an oxygen trim controller for adjusting the excess oxygen level in response to the sensed excess oxygen in the flue gas. Typically, the pressure controller and the oxygen trim controller are of the type commonly referred to PID controllers. Such controllers employ a control function having a proportional term, an integral term and a differential term. In conventional practice, once the commissioning process is completed, it is necessary for the commissioning technician to separately tune the oxygen trim controller and the pressure controller through trial and error method or further step tests. The purpose of the tuning process is to establish the gain factors associated with the proportional, integral and differential terms of the control function to provide a control function that is applicable over the entire firing range of the associated combustion system. The tuning of both controllers through further tests after completion of the commissioning process lengthens the time required for a technician to complete installation of the combustion control system.